This invention relates to a bridge amplifier, and particularly to a high power bridge amplifier suitable for use in motor vehicle music reproduction systems.
The demand for power audio amplifiers with more output power capability has been growing. However, when such amplifiers are set to avoid clipping of the dynamic extremes of a music signal, the average power is 10 to 20 times less than that needed for peak instantaneous power. In addition, an amplifier used in a vehicle music reproduction system must often fit in a very limited space, with consequent restrictions on the size of heat sinks and thus on average power dissipation. In order to include a high power amplifier in a vehicle music system, a more efficient amplifier configuration than the typical class B or AB would be advantageous.
A class G amplifier provides the efficient operation desired for minimization of heat sink requirements in a high power amplifier. In a class G amplifier, a first fixed supply voltage is used across the output section until the output voltage attempts to rise above a voltage somewhat below the first fixed supply voltage, whereupon a second, higher voltage power supply is switched in as needed to follow the amplified signal and allow the output voltage to increase. The superior efficiency of class G amplifiers, compared to class B, has been described in the literature, for example in the article "Highest Efficiency and Super Quality Audio Amplifier Using MOS Power FETS in Class G Operation" by Sampei, Ohashi, Ohta and Inoue, IEEE Transactions on Consumer Electronics Vol. CE-24(3), p. 300-306, Aug., 1978. Class G amplifiers have been used mainly in home applications, however, since they generally use both positive and negative power supplies at two voltages. Such power supplies are easily obtained through transformers from home 115 volt AC power; but the low voltage, unipolar, DC supply of a motor vehicle presents a challenge. Class G bridge amplifiers continue to remain attractive for high power, low space applications, however, due to their lower part content as well as their higher efficiency. Thus, a class G bridge amplifier capable of operation from unipolar supplies is desirable.